CKD is the particulate matter carried from the feed end of a cement (clinker-producing) kiln (industrial furnace) by the exhaust gases resulting from combustion and calcination. CKD typically consists of particles of raw materials, partly to wholly calcined raw materials, reaction process intermediates, fine clinker, and inorganic fuel solids; it usually also includes condensates and reaction products from the inorganic raw material and fuel volatiles.
A particular CKD can vary in composition (see Table 1) from virtually unaltered kiln feed (raw meal) to over 90% alkali sulfates and chlorides (Haynes and Kramer, "US Bureau of Mines Circular 8885," 1982; and personal observation), depending on process type, kiln configuration, raw materials, fuel(s), process characteristics, and point(s) of collection. It can vary in particle size from that of fine sand or silt to that of clay, with particle size distribution ranging from very broad to very narrow depending on material and process parameters.
TABLE 1 ANALYSES SHOWING EXTREMES OF CKD COMPOSITION (Relative to Kiln Feed) OXIDE MINIMUM CHANGE* MAXIMUM CHANGE SiO.sub.2 12.11 1.07 Al.sub.2 O.sub.3 4.44 0.28 Fe.sub.2 O.sub.3 1.44 0.24 CaO 47.29 (43.0) 2.19 MgO 1.08 0.10 LOI 35.00 1.50 Na.sub.2 O 0.12 4.36 K.sub.2 O 1.54 43.31 SO.sub.3 1.15 45.96 Total 103.77 99.01 *Analysis #63 in Haynes and Kramer. The total indicates this analysis to be of low quality. CaO analysis is higher by about 5% and SiO.sub.2 lower by about 2% relative to typical expectation for kiln feed.
The quantity of dust generated from a particular kiln depends on the factors noted above as affecting composition and particle size, the internal configuration of the kiln, the quantity of gases passing through the kiln, and other operating conditions. In general, the amount of dust generated from a kiln system ranges from about 5% to over 20% of kiln feed, with an average "emission factor" of around 12% ("Emission Factors for Industrial Processes," EPA publication AP-42) of clinker produced; EPA indicates relatively little dependence upon process types.
United States clinker capacity of 75 to 80 million tons per year has remained relatively steady for the past 20 years ("US and Canadian Portland Cement Industry: Plant Information Summary, December 1995"; Portland Cement Association, Skokie, Ill., November 1996). Applying the EPA Emission Factors suggests that about 9 to 10 million tons of CKD are generated per year; that about 300 million tons have been generated over the past 30 years; and that over 1 billion tons have been generated since the start of the US cement industry over a century ago.
Kiln dust is a major problem at many cement manufacturing plants. Dust is generated in large quantities and is often not suitable for direct return to the cement-producing process as a feed, except in relatively small amounts, often because of high concentrations of alkalies and sulfates, because of incompatibility of the dust with the process, or because of limitations of the process equipment. It is estimated that less than half of the dust generated each year is returned to the process. (Some plants return all of their dust while others return none of it.) Since large quantities of dust cannot be returned directly to the kiln in all cases, that material must be disposed in some manner. Beneficial uses have been suggested, but the amounts of dust so used are relatively small. For some uses, only a few dusts are acceptable, while for others, such as agricultural liming, use is seasonal, but generation is continuous. Frequently, dust that cannot be directly returned or reused is discarded in waste piles, or it may be placed in land- or quarry-fills. Such disposal methods are inherently unsatisfactory because they involve wasting a material for which significant processing and handling costs and effort have been incurred. In addition, as environmental regulations have matured, the costs and problems of disposal have become more onerous, and continued disposal of kiln dust has become ever less desirable and more expensive.